Recently, white (white-light-emitting) organic EL devices are being actively developed because they can be used for a mono-color display device, a lighting apparatus such as a back light, and a full-color display with color filters. Chromaticity change in the white organic EL device not only impairs the quality of the product, but also causes decrease of color reproducibility in, for instance, a full-color display which is combined with color filters. Therefore, minimal chromaticity change is required for the white organic EL devices.
Many methods of producing white light emission by an organic EL device have been proposed. Few of the methods produce white light with only one kind of emitting material and a single organic EL device generally uses two or more kinds of emitting materials that emit light simultaneously. In the case of using three kinds of emitting materials, a combination of red, blue and green lights corresponding to three primary colors gives white light. However, there are problems that such organic EL devices have difficulty controlling chromaticity and that they are poor in reproducibility. In the case of using two kinds of emitting materials, a blue emitting material and a yellow-to-red emitting material, yellow-to-red being the complementary color to blue, are selected. However, the yellow-to-red light emission becomes dominant in many cases, and chromaticity change tends to occur. For instance, as shown in reference examples 1 and 2 of Patent Document 1, blue emission tends to be lowered in conventional white organic EL devices and they have problem in the point of chromaticity change. Further, a blue dopant and a yellow-to-red dopant are used together and the doping ratio thereof can be adjusted to give white light emission. However, in addition to the tendency for red to become dominant, energy is liable to transfer from blue to red so as to give reddish white light. Therefore, in order to obtain white light, a yellow-to-red dopant has to be used in a considerably diluted manner, so that there is a problem in the point of reproducibility.
In another method, a yellow-to-red emitting material is doped into the hole-transporting layer adjacent to the emitting layer. In this method, electrons are not easily injected into the hole-transporting layer so that even when doping with a yellow-to-red emitting material toward which the light emission tends to shift is conducted, strong red light emission does not occur. Thus, this method has advantages in that it is easy to balance the blue light emission and the yellow to red light emission in order to obtain white light emission and that the resultant organic EL device is excellent in luminous efficiency and has a long lifetime. However, in view of the problem that energy transmission depends upon distance, there has been a critical issue that extensive chromaticity change is observed after the organic EL device has been driven continuously or it has been kept under a high temperature condition. It is the inventors' understanding that the balance of electrons and holes breaks down with degeneration because excited red emitting molecules concentrate at the interface of the hole-transporting layer side, and even if the extent of the concentration to the interface changes slightly, red light emission changes significantly while blue light emission does not change much. This is the cause of chromaticity change.
Further, there is a stacked type organic EL device which has two emitting layers and in which the emitting layer at the anode side is a yellow-to-red emitting layer and the one at the cathode side is a blue emitting layer. Although this type is excellent in efficiency, it requires control of the yellow to red light emission in order to obtain white light emission. For this, it is necessary for the thickness of the yellow-to-red emitting layer to be made thinner than that of the blue emitting layer, or for the doping concentration of the yellow-to-red emitting material to be lowered, so that fabrication of such a device is difficult. Concretely, unless the thickness of the yellow-to-red emitting layer is made approximately 1 to 2 nm, white light emission cannot be obtained in many cases. Such a thickness is almost the same as the size of the molecules used in typical low molecular type organic EL devices, and it is very difficult to control the thickness to this value.
On the other hand, by making the emitting layer on the anode side, toward which color range of the emitted light is liable to shift, a blue emitting layer, the tendency of the emitted light color to shift to red is counteracted, white light emission can be obtained and chromaticity change during driving is less, even when the yellow-to-red emitting layer is given a thickness of approximately 10 to 30 nm. However, in view of practical use, a stable white organic EL device is desired whose chromaticity change is even smaller.
Patent Document 1: JP-A-2001-52870
An object of the invention is to provide an organic EL device which is stable and small in chromaticity change, can be driven with low voltage and has high efficiency.